Mastic applicators are known which are designed for applying mastic to joints between wall board or drywall panels. When two adjacent panels are in the same plane, their abutting edges form a flat wall joint but when two adjacent panels are not in the same plane, such as being at right angles, e.g. edges of the panels forming an outside corner, a flat surface applicator is not suitable and an applicator for negotiating the corner is required to facilitate application of mastic at an outside corner joint.
U.S. Pat. No. 5,814,351 describes an outside corner mastic applicator which has retractable corner guides with guide surface projections thereon. The guides are movable between a retractable position when the applicator is used on a flat surface and a deployed position when used on an outside corner and the guide surface projections are intended to guide the applicator along the outside corner. The applicator also includes a pair of rollers on its underside in order to support the applicator in relation to the surface. The rollers are mounted on the opposite ends of an axle that is pivotable about its centre. The rollers also each have a notch that is intended to engage with the outside corner bead.
However, in practice it has been found difficult to use the applicator on corners as an efficient means for guiding the roller to the corner is lacking. Also when using the applicator, it is often necessary to repeat the process one or more times and because the rollers are in contact with the wall, they create a line or track in the mastic. Therefore, the rollers tend to detract from the finish of the joint.
Since the rollers are mounted on the same axle, a problem that occurs is that if one roller touches the wall, the opposite roller also moves, rendering it difficult to align the applicator with the corner trim.
Because the rollers are mounted in a vertical position, it limits or hampers the applicator's ability to be used on radius corners or bulkheads that have radiuses. It requires side pressure to push the applicator against a corner bead as well as moving the applicator along the corner bead. On an inside radius, the roller binds and tends to run up the corner trim. On long runs of corner bead there usually are many joints in the bead where there is often misalignment of the bead causing the roller and the guide surface projections to catch on the misaligned bead.
When the corner guides are in the deployed position, the guide surface projections rub against the surface of the wall board often causing damage to the paper surface. The friction caused by this tends to turn the applicator slightly sideways causing the roller to come away from the corner bead and lose its guidance.
The applicator further requires the use of a fixed handle. However, when using this handle, a small movement sideways causes the roller to come off the corner, which results in difficulties in controlling the applicator.
When deploying mastic to a ceiling or a wall, the guide surface projections can damage the drywall causing the projections to catch on a lip between two adjacent sheets of wall board. This happens especially if the wallboard has been installed with uneven joints. In order to prevent this, it is necessary manually to adjust the guide surface projections to the retracted position.
It is, therefore, desirable to provide an applicator that overcomes or alleviates the above difficulties.